Q: What can I use besides just paint to build up impasto on the surface of my oil paintings?
A: Alkyd-based pastes and gel mediums will allow you to
create impasto when mixed with oil paint. These contain inert silica to
add strength and stiffness to the dried paint film. They also speed up
drying time--an important characteristic considering the slow drying
time of a traditional heavy oil impasto, which is also susceptible to
cracking when dry. Acrylic modeling paste may be used as well, but you
must wait until it is completely dry before painting over it in oil.
Another possibility is a wax medium. Remember that a heavy impasto will
require a rigid surface--a stretched canvas may crack when an impasto
surface dries.